Biomass as an energy source is attractive for two main reasons. First, it is renewable. Second, the carbon which makes up the backbone of it's structure is drawn from the air, lowering the carbon dioxide levels which are currently causing great concern for the environment.
Unfortunately, the energy density of biomass is low. It is a bulky commodity which can be expensive to handle and transport. Often the cost of transportation is more than the value of the energy in the biomass. That is why biomass as an energy source is not currently cost competitive with other options available on the energy market.
Numerous methods, such as baling, pelleting, grinding, gasifying, and liquefying have been used over the years to pack the energy content of biomass into a more easily transportable form. Unfortunately, most of the previously tried methods have not gained significant acceptance. Liquefaction is the method which is currently receiving the most attention.
The oldest, and most widely used method of liquefying biomass is the ancient art of fermentation. In the 1800's fermentation was expanded beyond just sugars to include fermenting cellulosic ethanol. Cellulosic ethanol was produced in Europe and America during the early 1900's, but it proved to be unprofitable and was eventually abandoned. Today, recent advances in fermentation have caused a renewed look at this ancient art. But, fermentation is a slow process and ethanol facilities must be large to be economically viable. These large facilities consume more biomass than can be produced in the close surrounding area. In some instances their feedstock must be transported in over long distances. That is why one of the biggest expenses in cellulosic ethanol production has always been the cost of transporting biomass to large fermentation facilities.
A more recently discovered liquefaction process is fast pyrolysis. Brown (2003:182-183) says, “Fast pyrolysis is the rapid thermal decomposition of organic compounds in the absence of oxygen to produce liquids, gases, and char. The distribution of products depends on the biomass composition and rate and duration of heating. Liquid yields as high as 78% are possible for relatively short residence times (0.5-2 s), moderate temperatures (400-600 C.), and rapid quenching at the end of the process.”
Fast pyrolysis can liquefy a lot of biomass with a relatively small processor. In fact, the invention we disclose in this patent was made small enough that a one ton per day prototype is currently in use as a small scale, trailer mounted, transportable pyrolysis processor. This processor can be taken directly to the site where biomass is produced, eliminating the cost of transporting biomass altogether.
In the past, fast pyrolysis oil never gained acceptance as an energy source because it was of low quality, and had little value as a fuel. The molecules which made up fast pyrolysis oils contained large amounts of oxygen in their structure. This made them unstable and gave them a relatively low heating value compared to petroleum based oils. They needed to be upgraded and hydrogenated to make high grade fuel. Unfortunately, fast pyrolysis oil was also very acidic, having high concentrations of organic acids. This acidity meant that normal refinery equipment would be quickly damaged while trying to upgrade typical fast pyrolysis oil.
Using Calcined Limestone in Pyrolysis Reactions
For many decades people have made use of calcined limestone, or more accurately calcined calcium carbonate (CaCO3), in various ways to achieve a desired result from their pyrolysis inventions. In all these inventions, the reaction conditions were tailored to promote those aspects of the calcining/carbonation cycle which the inventor felt would be useful.
Calcining is a reversible chemical reaction that occurs when CaCO3 is heated above a temperature which is heavily dependent on the level of carbon dioxide (CO2) in the atmosphere. When CaCO3 calcines, it absorbs heat, gives off CO2, and turns into calcium oxide (CaO). In normal air this begins at about 550° C. In a CO2 rich atmosphere, such as is found in a calcining chamber, the temperature will be much higher. For calcining to happen at a useful rate inside a calcining chamber the temperature must be greater than about 900° C.
When the temperature of CaO is then lowered, it absorbs CO2, releases its stored heat and turns back into CaCO3.
This release of CO2 at high temperatures and the re-absorption of CO2 at low temperatures makes CaCO3 an ideal heat carrier for our pyrolysis reactor.
In the past, most inventions which made use of the calcining/carbonation cycle of CaCO3, were aimed at gasifying carbonaceous solids or extracting CO2 from a gaseous mixture. Many of these gasifiers also made use of the reaction C(s)+H2O(g)→CO+H2 to draw carbon out of the material they sought to gasify and add to the yield of hydrogen gas produced.
In 1915, Georges Claude received U.S. Pat. No. 1,135,355 for a process to produce relatively pure hydrogen by extracting carbon monoxide from Water Gas with calcium hydroxide, which is the hydrated form of calcined lime. This invention used CO absorption, but made no use of the exothermic nature of the absorption.
In 1953, Everett Gorin received U.S. Pat. No. 2,705,672 for his process which used steam and calcium oxide to produce Water Gas from carbonaceous solids in a high temperature, high pressure reaction.
In 1982, Shang-I Cheng received U.S. Pat. No. 4,353,713 for his invention to gasify coal. His invention injects water into the reaction to consume carbon by way of the C(s)+H2O(g)→CO+H2 reaction, and it injects CO2 from the combustion phase to keep CO2 levels high in the pyrolysis zone. His invention used the exothermic heat of the CO2 absorption process to drive the pyrolysis reaction.
In 2004, Klaus S. Lackner received U.S. Pat. No. 6,790,430 for his invention to produce hydrogen from carbonaceous material. This invention is much more involved than Mr. Cheng's or Mr. Gorin's, but it still makes use of water injection to help consume carbon and increase the hydrogen yield.
A survey of the many patents that use the calcining/carbonation cycle of CaCO3, shows that water or steam is usually added to increase the production of hydrogen and consume carbon by the reactions:C(s)+H2O(g)→CO+H2 followed byCO+H2O(g)→CO2+H2 
Also, in most of those previous patents, CO2 levels were kept high to get more process heat from the exothermal carbonation of CaO.